1. Field of Invention
The invention relates to a driving control device for an electric vehicle, a driving control method for an electric vehicle, and a program thereof.
2. Description of Related Art
Conventionally, a driving device for a vehicle is mounted on an electric automobile, as an exemplary electric vehicle, generates torque of a motor as an electric machine, that is, motor torque, and transmits the motor torque to driving wheels. In such a driving device for a vehicle, the motor is driven by a direct current received from a battery at the time of power running (driving), generates the motor torque, receives the torque by inertia of the electric vehicle at the regeneration (power generation) time, generates a direct current, and supplies the current to the battery.
Also, a driving device for a vehicle is mounted on a hybrid vehicle, as an electric vehicle, and transmits a part of engine torque to a power generator (a generator motor) as a first electric machine, and the remaining engine torque to the driving wheels. Such a driving device for a vehicle, including a planetary gear unit which includes a sun gear, a ring gear and a carrier, couples the carrier with the engine, couples the ring gear with the driving wheels, couples the sun gear with the power generator, and transmits rotation which is output from the ring gear and a motor as a second electric machine to the driving wheels to generate a driving force.
A motor control device is arranged in each of the above-mentioned driving devices for the vehicle. In each of the motor control devices, a direct current from the battery is converted into an alternating current by an inverter, and the alternating current is supplied to the motor. Then, the electric vehicle is made to run by driving the motor. In this case, feedback-control by vector control computation is performed on a d-q axis model whose d-axis is in a direction of paired magnetic poles of a rotor of the motor, and q-axis is in a direction at right angles to the d-axis. However, when the motor is driven at a high speed during the feedback-control, an induced voltage rises, which prevents a motor rotational speed from being increased, and narrows a power range of the motor. Accordingly, when the motor is being driven at a high speed, a magnetic flux amount, which is generated from a permanent magnet in a magnetic field that is formed by a d-axial current, is reduced to perform field weakening control.
However, in the conventional driving device for a vehicle, when the field weakening control becomes unperformable for some reason, a high counter electromotive voltage is generated, and an overcurrent is thereby supplied to the battery. As a result, the battery is overcharged. Potential causes include the following: the motor control device becomes incapable of controlling the motor due to a disturbance by noise, physical damage or the like; and the inverter or at least one line among three lines for supplying a current from the inverter to the motor is physically damaged. Generally, because a battery with relatively high voltage, such as 144, 288, 312 V or the like, is used for the electric vehicle or the hybrid vehicle, a current supplied to the battery is low. Therefore, when an overcurrent is about to be supplied to the battery, a relay on a direct current cable for supplying a direct current from the battery to the inverter is opened to prevent the overcurrent from being supplied to the battery.
In contrast to this, for example, in the case of the electric vehicle or the hybrid vehicle in which a battery with a relatively low voltage, such as 42 V or the like, is used, a current supplied to the battery is larger by an increased amount of current with respect to the battery with high voltage. Accordingly, in the case where an overcurrent is about to be supplied to the battery, when the relay on the direct current cable needs to be opened, the relay must be upsized. As a result, not only the driving device for a vehicle is upsized but also an arcing phenomenon occurs, which increases a period of time until the relay is opened.
In addition, even in the case where the field weakening control or the like is properly performed, when the battery and the inverter are electrically insulated because the direct current cable is broken, a connection terminal of the direct current cable is detached or the like, a voltage drop by an amount of internal resistance of the battery ceases, and an excessively high voltage is applied to the inverter.
Also, when a power transmission line for supplying power to auxiliaries of the electric vehicle, such as an electrical component, branches off from the direct current cable, and is connected to the electrical component, an excessively high voltage is applied to the electrical component.